A passive optical network (PON) is a point-to-multipoint network fiber architecture for providing broadband services to business and residential subscribers. A PON comprises an optical transmission line (e.g., a fiber or optical link) coupled to an upstream node, such as an optical line terminal (OLT) located at service provider premises or central office (CO), and coupled to a downstream node, such as an optical network units (ONU), located at service subscriber or end-user premises.
A PON may implement a variety of topologies, such as tree, ring, or bus topologies. In a tree topology, one optical fiber couples the OLT to a passive optical splitter, which distributes downstream optical signals from an OLT to ONUs and combines upstream optical signals from ONUs to the OLT. The splitter merges or multiplexes upstream communications onto the shared optical link. A splitter may use a time division multiple access (TDMA) protocol where packet communications from ONUs are assigned time slots for transmission to the OLT. Alternatively, a splitter may use a distinct frequency for each ONU, such as in wavelength division multiplexing (WDM). An OLT may couple an optical access network to an external network, such as a metro backbone belonging to an internet service provider (ISP) or a local exchange carrier.
Communication in a PON comprises upstream traffic transmitted from ONUs to an OLT and downstream traffic transmitted from the OLT to the ONUs. ONUs share channel capacity and resources provided by the shared optical link coupling the passive optical splitter and the OLT. The passive optical splitter may divide (separate) and distribute downstream communications (optical signals) from the OLT to each ONU or may broadcast downstream communications transmitted by the OLT to all ONUs where each destination ONU extracts data frames from communications based on Logic Link Identifiers (LLIDs) assigned to each ONU. An LLID establishes a logical link between an ONU and OLT, which accommodates specific service level agreement (SLA) requirements. An LLID carries physical address information for a frame to determine which ONU is allowed to extract the frame.
An Ethernet passive optical network (EPON) combines Ethernet technology with inexpensive passive optics. An EPON offers the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. In particular, due to the high bandwidth of optical fibers, EPONs are capable of accommodating broadband voice, data, and video traffic simultaneously.
Each ONU can accommodate a number of networked devices, such as personal computers, telephones, video equipment, network servers, etc. One or more networked devices belonging to the same class of service are typically assigned an LLID.
Because a PON adopts passive optical transmission technology, which excludes signal amplification and regeneration, a PON is subject to power limitations and various transmission impairments. As a result, the signal-to-noise ratio of a PON suffers as the network increases in size, resulting in more frequent bit errors. Fortunately, forward error correction (FEC) techniques can mitigate these undesirable effects and can help increase the power budget.
FEC is a transmission error correction technique allowing a receiving device to detect and correct errors. A receiving device, such as each ONU and each OLT in a PON, utilizing FEC has the capability to detect and correct any block of symbols that contains fewer than a predetermined number of error symbols. A transmitting device supports FEC by adding bits to each transmitted symbol block, using a predetermined error correction technique. One exemplary technique is the use of Reed-Solomon code. A Reed-Solomon code is specified as RS(1, k) with s-bit symbols, which means that the encoder takes k data symbols of s bits each, and adds (1-k) parity symbols to make an 1-symbol codeword. A Reed-Solomon decoder can correct up to t symbols that contain errors in a codeword, where 2t=1-k. For example, RS(255, 239) with 8-bit symbols means that each codeword contains 255 bytes, of which 239 bytes are data and 16 bytes are parity. The decoder can automatically correct errors contained in up to 8 bytes anywhere in the codeword.
Communication protocols vary between 1 Gbps and 10 Gbps EPON standards. According to the 10 Gbps EPON standard (10G-EPON), two connection configurations are supported, including “symmetric,” which operates at a 10 Gbps data rate in both upstream (customer to provider) and downstream (provider to customer) directions, and “asymmetric,” which operates at 10 Gbps in the downstream direction and 1 Gbps in the upstream direction. 10G-EPON supports line encoding. According to the 1 Gbps EPON standard (1G-EPON), 1 Gbps data rates are supported in the upstream and downstream directions. In a 1 Gbps EPON, an FEC-coded Ethernet frame may begin with a start FEC (SFEC) code sequence. Following SFEC is an Ethernet frame, which includes a preamble/start-of-frame delimiter (SFD) field, a data frame, and a frame-check-sequence (FCS) field. FCS field typically contains a cyclic redundancy check (CRC) sequence. Following FCS field is an end-of-frame delimiter terminate FEC (TFEC) indicating the end of the Ethernet frame. TFEC may delineate the Ethernet frame from subsequent FEC parity bits.
A serializer/de-serializer (SerDes) is an integrated circuit (IC or chip) transceiver that converts parallel data to serial data and vice-versa. A SerDes may be incorporated into each OLT and ONU in a PON. A transmitter section of a SerDes has parallel data input and serial data output. For example, a transmitter section of a SerDes in a gigabit Ethernet system may include 10 parallel input data lines clocked at 125 MHz and serial data output lines clocked at 1.25 GHz. A receiver section of the SerDes is the reverse of the transmitter section, i.e., serial data input and parallel data output. The receiver section recovers a clock embedded in a received signal for use in the decoding process. A gigabit Ethernet SerDes may use an 8B/10B coding scheme that maps 8-bit symbols to 10-bit symbols to achieve DC-balance on a transmission line.
A SerDes may require calibration for communication channel attributes, such as offsets, etc. Traditional calibration uses comparators and accumulators to adjust SerDes settings. A comparator receives the same input as a SerDes component to compare to a threshold while an accumulator counts the number of comparator events greater or less than the threshold for use in adjusting the component. Traditional calibration is done without regard to the accuracy of received bits relative to transmitted bits. The characteristics of transmitted data bits and patterns, e.g., frequency, edge, and errors in received characteristics, are not factored into calibration. Thus, traditional calibration is coarse, incapable of making fine adjustments. As a result, bit error rate (BER) may remain unacceptably high. A substantial quantity of input comparators may also increase the capacitive load of the input circuit.